Effect of plant-self debris on nitrogen removal, transformation and microbial community in mesocosm constructed wetlands planted with Myriophyllum aquaticum.

Aquatic macrophytes debris decomposition could release pollutants and nutrients into the water of constructed wetlands (CWs), but their role in nitrogen removal and transformation remains poorly understood. The present study investigated the effects of plant-self debris on nitrogen removal and microbial communities in mesocosm CWs planted with Myriophyllum aquaticum. During the 68-day operation, the plant debris addition did not change the mean removal efficiency of ammonium (NH4+-N) and total nitrogen (TN) of CWs but showed significant differences over the operation time. The NH4+-N and organic nitrogen released from the plant debris decomposition affected the nitrogen removal. The plant debris decreased the effluent nitrate concentration and N2O emission fluxes of the CWs with the increased abundance of denitrifying bacterial genera, indicating that plant debris decomposition increased the denitrification activities via dissolved organic carbon release. High-throughput sequencing indicated that the plant debris altered the distribution and composition of the microbial community in the sediments. Proteobacteria was the dominant phylum (28-52%), and the relative abundance of denitrifying bacteria genera was significantly higher in the sediments with debris addition (37-40%) than in the non-addition (6.6-7.7%). The present study provided new insights into the role of macrophytes in pollutant removal and the plant management strategy of CWs.

Physiological response and tolerance of Myriophyllum aquaticum to a wide range of ammonium concentrations.

Myriophyllum aquaticum (M. aquaticum) can be used in constructed wetlands (CWs) to effectively purify swine wastewater with high-ammonia nitrogen (NH3-N and NH4+-N) concentrations. However, the understanding of its tolerance mechanism to ammonia nitrogen is limited. The physiological response and tolerance mechanism of M. aquaticum to a wide range of NH4+ concentrations (0-35 mM) were investigated in the present study. The results indicated that M. aquaticum can tolerate NH4+ concentrations of up to 30 mM for 21 days and grow well with high nutrient (N, P) uptake. A suitable concentration of NH4+ for a better growth of M. aquaticum was 0.5-20 mM. The free NH4+ content was no obviously increase at NH4+ concentration below 15 mM, indicated there was no obviously ammonium accumulation. Exogenous NH4+ inhibited K+ absorption and improved Ca2+ absorption, indicating mineral cation could mediate NH4+ homeostasis under NH4+ stress. Moreover, comparison with those in the control group, the activities of glutamine synthetase (GS), glutamate synthetase (GOGAT) in M. aquaticum increased by 52.7%-115% at 1-20 mM NH4+, and superoxide dismutase (SOD) increased by 29.2-143% at 1-35 mM NH4+. This indicated that the high NH4+ tolerance of M. aquaticum was mainly due to the balance of free NH4+ content in tissues, as well as improved nitrogen metabolism and antioxidant system. This could be attributed to the role of the GS-GOGAT cycle and SOD. In conclusion, M. aquaticum, which tolerates high NH4+ concentration and has a high N uptake ability, can be used as a good candidate specie to help develop more efficient management strategies for treating high-NH4+ wastewater in CW systems.

Defect Engineering on a Ti4O7 Electrode by Ce3+ Doping for the Efficient Electrooxidation of Perfluorooctanesulfonate.

Defect engineering in an electrocatalyst, such as doping, has the potential to significantly enhance its catalytic activity and stability. Herein, we report the use of a defect engineering strategy to enhance the electrochemical reactivity of Ti4O7 through Ce3+ doping (1-3 at. %), resulting in the significantly accelerated interfacial charge transfer and yielding a 37-129% increase in the anodic production of the hydroxyl radical (OH•). The Ce3+-doped Ti4O7 electrodes, [(Ti1-xCex)4O7], also exhibited a more stable electrocatalytic activity than the pristine Ti4O7 electrode so as to facilitate the long-term operation. Furthermore, (Ti1-xCex)4O7 electrodes were also shown to effectively mineralize perfluorooctanesulfonate (PFOS) in electrooxidation processes in both a trace-concentration river water sample and a simulated preconcentration waste stream sample. A 3 at. % dopant amount of Ce3+ resulted in a PFOS oxidation rate 2.4× greater than that of the pristine Ti4O7 electrode. X-ray photoelectron spectroscopy results suggest that Ce3+ doping created surficial oxygen vacancies that may be responsible for the enhanced electrochemical reactivity and stability of the (Ti1-xCex)4O7 electrodes. Results of this study provide insights into the defect engineering strategy for boosting the electrochemical performance of the Ti4O7 electrode with a robust reactivity and stability.

Nitrogen removal performance and needed area estimation of surface-flow constructed wetlands using a probabilistic approach.

Pollutant concentrations in influents into constructed wetlands (CWs) are highly fluctuating and may vary over several orders of magnitude, leading to large uncertainties in removal performance assessment when using pollutant concentrations in the influent and effluent directly. Incorporating a probabilistic approach into removal performance assessment and needed area estimation of CWs could advantage decision making regarding wastewater treatment and engineering applications. A series of three-stage surface-flow CWs (SFCWs) were constructed for treating ammonium-rich swine wastewater. The surface removal rate and removal efficiency of ammonium nitrogen in the SFCWs using the probabilistic approach were 0.27-3.23 g m-2 d-1 and 43.0-99.9% (95% confidence interval (CI)), which were consistent with the deterministic approach (95% CI: 0.24-3.18 g m-2 d-1 and 70.4-99.9%). The needed SFCW area was estimated as 6.6 (95% CI: 1.4-17.8) to 29.7 (95% CI: 6.4-80.1) m2 for required removal efficiency from 40% to 90% for 0.18 m3 d-1 swine wastewater with different strengthens. For specific removal efficiency of 90%, the needed CW areas was 13.9 (95%CI: 4.9-42.7), 25.1 (95%CI: 5.9-66.0), 33.5 (95%CI: 13.5-87.1), and 40.8 (95%CI: 16.2-89.4) m2 for influent ammonium loading rate of 0.18-2.7, 2.7-14.4, 14.4-36, and 36-60 g d-1, respectively. The first-order removal constant of ammonium nitrogen decreased logarithmically with increasing influent and effluent concentration/loading rate in the SFCW units (p < 0.001), which was responsible for the needed SFCW areas covering a wide range. The reliability analysis confirmed the results from the probabilistic approach were appropriate. The present study shed new lights on the performance evaluation and design of CWs for treating wastewater with highly-fluctuating concentrations using a probabilistic approach.

Anaerobic ammonium oxidation in sediments of surface flow constructed wetlands treating swine wastewater.

Anaerobic ammonium oxidation (anammox) was suggested to be involved in the nitrogen (N) removal process in constructed wetlands (CWs). Nevertheless, its occurrence and role in CWs treating swine wastewater have not been well evaluated yet. In this study, we investigated the diversity, activity, and role of anammox bacteria in sediments of mesoscale surface flow CWs (SFCWs) subjected to different N loads of swine wastewater. We found that anammox bacteria were abundant in SFCW sediments, as indicated by 7.5 × 105 to 3.5 × 106 copies of the marker hzsB gene per gram of dry soil. Based on stable isotope tracing, potential anammox rates ranged from 1.03 to 12.5 nmol N g-1 dry soil h-1, accounting for 8.63-57.1% of total N2 production. We estimated that a total N removal rate of 0.83-2.68 kg N year-1 was linked to the anammox process, representing ca. 10% of the N load. Phylogenetic analyses of 16S ribosomal RNA (rRNA) revealed the presence of multiple co-occurring anammox genera, including "Candidatus Brocadia" as the most common one, "Ca. Kuenenia," "Ca. Scalindua," and four novel unidentified clusters. Correlation analyses suggested that the activity and abundance of anammox bacteria were strongly related to sediments pH, NH4+-N, and NO2--N. In conclusion, our results confirmed the presence of diverse anammox bacteria and indicated that the anammox process could serve as a promising N removal pathway in the treatment of swine wastewater by SFCWs.

Nitrogen removal in Myriophyllum aquaticum wetland microcosms for swine wastewater treatment: 15 N-labelled nitrogen mass balance analysis.

BACKGROUND: Ecological treatments are effective for treating agricultural wastewater. In this study, wetland microcosms vegetated with Myriophyllum aquaticum were designed for nitrogen (N) removal from two strengths of swine wastewater, and 15 N-labelled ammonium (NH4+ -N) was added to evaluate the dominant NH4+ -N removal pathway. RESULTS: The results showed that 98.8% of NH4+ -N and 88.3% of TN (TN: 248.6 mg L-1 ) were removed from low-strength swine wastewater (SW1) after an incubation of 21 days, with corresponding values for high-strength swine wastewater (SW2) being 99.2% of NH4+ -N and 87.8% of TN (TN: 494.9 mg L-1 ). Plant uptake and soil adsorption respectively accounted for 24.0% and 15.6% of the added 15 N. Meanwhile, above-ground tissues of M. aquaticum had significantly higher biomass and TN content than below-ground (P < 0.05). 15 N mass balance analysis indicated that gas losses contributed 52.0% to the added 15 N, but the N2 O flux constituted only 7.5% of total gas losses. The dynamics of NO3- -N and N2 O flux revealed that strong nitrification and denitrification occurred in M. aquaticum microcosms, which was a dominant N removal pathway. CONCLUSION: These findings demonstrated that M. aquaticum could feasibly be used to construct wetlands for high N-loaded animal wastewater treatment. © 2016 Society of Chemical Industry.

Nitrogen removal and mass balance in newly-formed Myriophyllum aquaticum mesocosm during a single 28-day incubation with swine wastewater treatment.

The aim of this research was to assess the applicability of Myriophyllum (M.) aquaticum for swine wastewater treatment. Nitrogen (N) removal processes were investigated in M. aquaticum mesocosms with swine wastewater (SW), 50% diluted swine wastewater (50% SW), and two strengths of synthetic wastewater, 200 mg [Formula: see text] L(-1) (200 [Formula: see text] ) and 400 mg [Formula: see text] L(-1) (400 [Formula: see text] ). During a 28-day incubation period, the average [Formula: see text] and TN removal rates were 99.8% and 94.2% for 50% SW and 99.8% and 93.8% for SW, which were greater than 86.5% and 83.7% for 200 [Formula: see text] , and 73.7% and 74.1% for 400 [Formula: see text] , respectively. A maximum areal total nitrogen (TN) removal rate of 157.8 mg N m(-2) d(-1) was found in M. aquaticum mesocosms with SW. During the incubation period, the observed dynamics of [Formula: see text] concentrations in water and gene copy numbers of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nirK and nirS in soil unraveled strong nitrification and denitrification processes occurring in M. aquaticum mesocosms with swine wastewater. The N mass balance analysis indicated that plant uptake and soil N accumulation accounted for 17.9-42.2% and 18.0-43.8% of the initial TN load, respectively. The coupled nitrification and denitrification process was calculated to account for, on average, 36.8% and 62.8% of TN removal for 50% SW and SW, respectively. These findings demonstrated that the N uptake by M. aquaticum contributed to a considerable proportion of N removal. In particular, the activities of ammonia-oxidizing and denitrification microbes responsible for nitrification and denitrification processes in M. aquaticum mesocosm accelerated [Formula: see text] and TN removal from swine wastewater.

Mitigating nitrous oxide emissions from tea field soil using bioaugmentation with a Trichoderma viride biofertilizer.

Land-use conversion from woodlands to tea fields in subtropical areas of central China leads to increased nitrous oxide (N2O) emissions, partly due to increased nitrogen fertilizer use. A field investigation of N2O using a static closed chamber-gas chromatography revealed that the average N2O fluxes in tea fields with 225 kg N ha(-1) yr(-1) fertilizer application were 9.4 ± 6.2 times higher than those of woodlands. Accordingly, it is urgent to develop practices for mitigating N2O emissions from tea fields. By liquid-state fermentation of sweet potato starch wastewater and solid-state fermentation of paddy straw with application of Trichoderma viride, we provided the tea plantation with biofertilizer containing 2.4 t C ha(-1) and 58.7 kg N ha(-1). Compared to use of synthetic N fertilizer, use of biofertilizer at 225 kg N ha(-1) yr(-1) significantly reduced N2O emissions by 33.3%-71.8% and increased the tea yield by 16.2%-62.2%. Therefore, the process of bioconversion/bioaugmentation tested in this study was found to be a cost-effective and feasible approach to reducing N2O emissions and can be considered the best management practice for tea fields.

Distinct effects of biochar addition on soil macropore characteristics at different depths in a double-rice paddy field.

Soil macropores largely control the water and nutrients transport as well as runoff processes in the soil. Biochar is frequently applied to soils to improve the macropore structure, but the effects remain controversial. To clarify depth-dependent soil macropore characteristics affected by biochar addition, the intact soil cores with a depth of 200 mm were collected from biochar-amended paddy field at addition rates of 0, 24, and 48 t ha-1 (CK, BC1, and BC2, respectively). The two biochar treatments did not change the overall soil pore indices (e.g., macroporosity, pore number, fractal dimension, and circularity), but showed distinct effects at different soil depths. At a soil depth of 0-50 mm, the biochar treatments had higher macroporosity (8.59-8.85 %) than CK (4.94 %) (p < 0.05), but relatively lower pore circularity (0.83-0.84) than CK (0.88) (p < 0.05). The connectivity of biochar treatments (88-97) was 9.5-10.4 times higher than that of CK (9.3). At a soil depth of 100-200 mm, the biochar treatments exhibited lower macroporosity, macropore number, connectivity, and fractal dimension than CK (p < 0.05). The macropore indices (except circularity) of BC1 were relatively higher than those of BC2 in the most soil depths. Whether biochar altered the soil macropore indices depended on the addition rate of biochar and soil depth. The expansion and occupying effects of biochar were dominant at soil depths of 0-50 and 100-200 mm, respectively; and the two effects were stronger in BC1 than in BC2. A combination of the expansion and occupying effects occurred at a soil depth of 50-100 mm. The distinct effects of biochar on soil pore structure at different depths could mitigate methane emission and nutrient runoff loss from the double-rice paddy. Therefore, soil depth-dependent macropore structure should be considered when assessing the influence of biochar on soil properties and the associated environmental effects.

Effects of water level on nitrous oxide emissions from vegetated ditches.

Nitrous oxide (N2O) is considered a powerful greenhouse gas. Vegetated ditches are an important source of N2O emissions in the agricultural systems. However, few studies have examined on the relationship between N2O emissions and the water level in vegetated ditches. To investigate the effect of water level on the N2O emissions, three pilot-scale ditches vegetated with Myriophyllum aquaticum were constructed with low (LW), medium (MW), and high (HW) water levels. The examined results indicated that the M. aquaticum ditches decreased N2O emissions by 38.4% and 67.9% in MW and HW, respectively, as compared to the LW ditch. In addition, the N2O emission factor decreased with increasing water level in the order of: LW (0.18%) > MW (0.11%) > HW (0.06%). The MW and HW ditches reduced the N2O emissions by controlling the sediment nitrogen contents, in which the ammonia nitrogen increased with increasing the level of water, while nitrate nitrogen decreased with increasing the level of water. The increase in the level of water significantly reduced the gene abundance of ammonia-oxidizing archaea (AOA) (p < 0.05), thereby reducing the N2O emissions in the MW and HW conditions due to the positive correlation between N2O emissions and AOA gene abundances. The unclassified_k_norank_d_Bacteria was the dominant denitrifying bacterial genus observed in the M. aquaticum ditches, and its highly relative abundance yielded low N2O emissions in the HW ditch. These findings indicate that reducing N2O emissions may be achieved by controlling the water level in vegetated ditches.

Glutamine synthetase plays an important role in ammonium tolerance of Myriophyllum aquaticum.

High-strength ammonium (NH4+), the main characteristic of swine wastewater, poses a significant threat to the rural ecological environment. As a novel phytoremediation technology, Myriophyllum aquaticum wetlands have high tolerance and removal rate of NH4+. Glutamine synthetase (GS), a pivotal enzyme in nitrogen (N) metabolism, is hypothesized to play an important role in the tolerance of M. aquaticum to high NH4+. Herein, the responses of M. aquaticum to GS inhibition by 0.1 mM methionine sulfoximine (MSX) under 15 mM NH4+ were investigated. After 5 days, visible NH4+ toxicity symptoms were observed in MSX-treated plants. Compared with the control, the NH4+ accumulation in the leaves increased by 20.99 times, while that of stems and roots increased by 3.27 times and 47.76 %, suggesting that GS inhibition had a greater impact on the leaves. GS inhibition decreased pigments in the leaves by 8.64 %-41.06 %, triggered oxidative stress, and affected ions concentrations in M. aquaticum. The concentrations of glutamine (Gln) and asparagine decreased by 63.46 %-97.43 % and 12.37 %-76.41 %, respectively, while the concentrations of most other amino acids increased after 5 days of MSX treatment, showing that GS inhibition reprogrammed the amino acids synthesis. A decrease in Gln explains the regulations of N-related genes, including increased expression of AMT in roots and decreased expression of GS, GOGAT, GDH, and AS, which would cause further NH4+ accumulation via promoting NH4+ uptake and decreasing NH4+ assimilation in M. aquaticum. This study revealed for the first time that GS inhibition under high NH4+ condition can lead to phytotoxicity in M. aquaticum due to NH4+ accumulation. The physiological and molecular responses of the leaves, stems, and roots confirmed the importance of GS in the high NH4+ tolerance of M. aquaticum. These findings provide new insights into NH4+ tolerance mechanisms in M. aquaticum and a theoretical foundation for the phytoremediation of high NH4+-loaded swine wastewater.

Unveiling the role of sediments in phosphorus removal in pilot-scale constructed wetlands for swine wastewater treatment.

The accumulation rate, fractions, and sorption capacity of phosphorus in sediments determine the removal efficiency and service life of constructed wetlands (CWs). Nine pilot-scale three-stage surface flow CWs were constructed to treat three loading rates of lagoon-pretreated swine wastewater, and surface sediment samples at initial and one-year treatment were collected to analyze the phosphorus fractions and sorption capacity. After one-year treatment, concentration of total phosphorus (TP) in sediments increased for high loading rates of wastewater, but remained stable for low loading rates. The annual accumulation rate of TP in sediments (Ma) was -43-445 mg kg-1 yr-1 at surface loading rate (SLR) of 36-355 g P m-2 yr-1. Their association could be described well using a sigmoid model, i.e., Ma = -23 + 538/(1 + exp.(-(SLR-262)/48)) (R2adj = 0.897, RMSE = 40.8, p < 0.01), indicating that the phosphorus accumulation rates in sediments were loading rate-dependent. The sum of inorganic phosphorus fractions contributed to 80-100% of the TP concentration, and accumulation of aluminum-bound phosphorus (AlP) and iron-bound phosphorus (FeP) was responsible for variability of TP concentration in sediments. Phosphorus sorption capacity of CW1 sediments increased by 1.3-1.8 times, attributed to increased pH, and concentrations of ammonium oxalate-extractable aluminum and iron in sediments due to the wastewater input. Selecting iron and aluminum-rich materials preferentially as substrates and regulating the ratio of metal ions to phosphorus in wastewater should be alternative enhancement strategies of CWs for phosphorus removal.

The immobilized Alcaligenes faecalis strain WT14 for removing high strength nitrate and reducing nitrite accumulation.

Microbial immobilization is considered as one of the effective denitrification techniques in the treatment of high load wastewater. In this study, the immobilized cells consisting of polyvinyl alcohol (PVA), sodium alginate (SA), and calcium chloride (CaCl2) were inoculated with Alcaligenes faecalis strain WT14 to treat wastewater with high nitrate-nitrogen (NO3--N) concentrations. After 48 h of wastewater treatment, 26.2-89.4% of total nitrogen (TN) was removed by the immobilized Alcaligenes faecalis strain WT14. The response surface methodology revealed the highest TN removal efficiency by Alcaligenes faecalis strain WT14 occurred at the immobilized ratio of 9.3% of PVA, 2.2% of SA and 1.9% of CaCl2. Under the optimal ratio of PVA, SA, and CaCl2, the conditions for the maximum denitrification efficiency and TN removal were pH of 7, temperature of 40°C, and shaking speed of 60 rpm·min-1. Compared to the free cells, the immobilization cells had no obvious negative effect on denitrification efficiency, additionally reduced the nitrite accumulation, and thus improved the TN removal. Furthermore, the immobilized cells still maintained 95.4% of NO3--N removal after the eighth cycle reuse. These results demonstrated the immobilized Alcaligenes faecalis strain WT14 can remove TN effectively and additionally reduce nitrite accumulation in treating high strength NO3--N wastewater.

Electro-activation of peroxymonosulfate by a graphene oxide/iron oxide nanoparticle-doped Ti4O7 ceramic membrane: mechanism of singlet oxygen generation in the removal of 1,4-dioxane.

Electro-activation of peroxymonosulfate (PMS) has been widely investigated for the degradation of organic pollutants. Herein, we employ graphene oxide (GO)/Fe3O4 nanoparticles (NPs) doped into a Ti4O7 reactive electrochemical membrane through strong chemical bonding as the cathode to activate PMS for the degradation of 1,4-dioxane (1,4-D). The strong chemical interaction between GO, Fe3O4-NPs, and Ti4O7 via Fe-O---GO---O-Ti bonds enhances the electron-transfer efficiency and provides catalytically active sites that boost the electro-activation of PMS. As a result, the 1,4-D oxidation rate of the GO/Fe3O4-NPs@Ti4O7 REM cathode is ~3 times higher (7.21 × 10-3 min-1) than those of other Ti4O7 ceramic membranes, and 1O2 plays a key role (59.9%) in the degradation of 1,4-D. The 1O2 generation mechanism in the electro-activation process of PMS was systematically investigated, and we claimed that 1O2 is mainly generated from the precursors H2O2 and O2•-/HO2• rather than by O2 or •OH, as has been reported in previous studies. A flow-through mode test in the PMS electro-activation system is firstly reported, and the 1,4-D decay efficiency is 7.1 times higher than that obtained by a flow-by mode, showing that an improved PMS mass transfer efficiency enhances the conversion to reactive oxygen species.

Interface engineering strategy of a Ti4O7 ceramic membrane via graphene oxide nanoparticles toward efficient electrooxidation of 1,4-dioxane.

Although Ti4O7 ceramic membrane has been recognized as one of the most promising anode materials for electrochemical advanced oxidation process (EAOP), it suffers from relatively low hydroxyl radical (•OH) production rate and high charge-transfer resistance that restricted its oxidation performance of organic pollutants. Herein, we reported an effective interface engineering strategy to develop a Ti4O7 reactive electrochemical membrane (REM) doped by graphene oxide nanoparticles (GONs), GONs@Ti4O7 REM, via strong GONs-O-Ti bonds. Results showed that 1% (wt%) GON doping on Ti4O7 REM significantly reduced the charge-transfer resistance from 73.87 to 8.42 Ω compared with the pristine Ti4O7 REM, and yielded •OH at 2.5-2.8 times higher rate. The 1,4-dioxane (1,4-D) oxidation rate in batch experiments by 1%GONs@Ti4O7 REM was 1.49×10-2 min-1, 2 times higher than that of the pristine Ti4O7 REM (7.51×10-3 min-1) and similar to that of BDD (1.79×10-2 min-1). The 1%GONs@Ti4O7 REM exhibited high stability after a polarization test of 90 h at 80 mA/cm2, and within 15 consecutive cycles, its oxidation performance was stable (95.1-99.2%) with about 1% of GONs lost on the REM. In addition, REM process can efficiently degrade refractory organic matters in the groundwater and landfill leachate, the total organic carbon was removed by 54.5% with a single-pass REM. A normalized electric energy consumption per log removal of 1,4-D (EE/O) was observed at only 0.2-0.6 kWh/m3. Our results suggested that chemical-bonded interface engineering strategy using GONs can facilitate the EAOP performance of Ti4O7 ceramic membrane with outstanding reactivity and stability.

Nutrients release and greenhouse gas emission during decomposition of Myriophyllum aquaticum in a sediment-water system.

Aquatic macrophytes play a significant role in nutrients removal in constructed wetlands, yet nutrients could be re-released due to plant debris decomposition. In this study, Myriophyllum aquaticum was used as a model plant debris and three debris biomass levels of 3 g, 9 g dry biomass, and 20 g fresh biomass (D3, D9, and F20, respectively) were used to simulate 120-d plant debris decomposition in a sediment-water system. The biomass first-order decomposition rate constants of D3, D9, and F20 treatments were 0.0058, 0.0117, and 0.0201 d-1, respectively with no significant difference of decomposition rate among three mass groups (p > 0.05). Plant debris decomposition decreased nitrate and total nitrogen concentrations but increased ammonium, organic nitrogen, and dissolved organic carbon (DOC) concentrations in overlying water. The parallel factor analysis confirms that three components of DOC in overlying water changed over decomposition time. Emission fluxes of methane and nitrous oxide in the plant debris treatments were several to thousands of times higher than the control group within the initial 0-45 d, which was mainly attributed to DOC released from the plant debris. Plant debris decomposition can affect the gas emission fluxes for relatively shorter time (30-60 d) than water quality (>120 d). The 16S rRNA, nirK, nirS and hazA gene abundance increased in the early stage for plant debris treatments, and then decreased to the end of 120-d incubation time while ammonia monooxygenase α-subunit A gene abundance of ammonia-oxidizing archaea and bacteria had no large variations during the entire decay time compared with no plant debris treatment. The results demonstrate that decomposition of M. aquaticum debris could affect greenhouse gas emission fluxes and microbial gene abundance in the sediment-water system besides overlying water quality.

Are vegetated drainage ditches effective for nitrogen removal under cold temperatures?

Vegetated ditches are widely used to treat agricultural wastewater, but effective nitrogen removal at low temperatures remains a challenge because plants wilt in the winter. In this study, three simulated drainage ditches vegetated with Myriophyllum aquaticum were operated with low, medium, and high water levels to study ammonium nitrogen (NH4+-N) removal under cold temperatures. The M. aquaticum ditches had a mean NH4+-N removal efficiency of 75.8-86.8% throughout cold period. Based on nitrogen mass balance, plant uptake, sediment adsorption, and microbial removal accounted for 12.4-21.5%, 0.0-8.1%, and 38.9-54.6% of the influent total nitrogen loading, respectively. The accumulation of nitrate confirmed that intense microbial nitrification occurred in M. aquaticum ditches even at low temperature. These results suggest that M. aquaticum is appropriate as a cold-tolerant plant for NH4+-N removal in drainage ditches.

Nitrous oxide emissions from pilot scale three-stage constructed wetlands with variable nitrogen loading.

Pilot scale three-stage surface flow constructed wetlands (SFCWs) were constructed to study nitrous oxide (N2O) emissions from swine wastewater with different nitrogen levels. The SFCWs had mean total nitrogen (TN) removal efficiencies and removal rates of 84.6-97.1% and 0.6-2.4 g N m-2 d-1 respectively. The N2O emissions and nitrate nitrogen (NO3--N) concentration both peaked at a TN value of approximately 100 mg N L-1. N2O emissions had a positive correlation with NO3--N concentration (p < 0.001). This correlation suggests that the effect of TN loading on N2O emissions may be related to NO3--N in aquatic environment. Significant correlation was observed between N2O emission and the gene abundance of N2O reductase (nosZ; p < 0.05). The general linear model revealed that TN loading affected nosZ gene abundance. These results suggest that pollution loading should be considered to balance nitrogen removal and N2O emissions when designing constructed wetlands.

Does rice straw application reduce N2O emissions from surface flow constructed wetlands for swine wastewater treatment?

Rice straw was applied often as a carbon source to improve nitrogen removal; however, few studies have considered the effect of rice straw on nitrous oxide (N2O) emission during nitrogen removal in constructed wetlands (CWs). We constructed eighteen combined systems, consisting of rice straw ponds and surface flow CWs to investigate the effect of rice straw application on N2O emission in three strengths of swine wastewater treatments. The results showed rice straw (RS) treatment increased 131.5% of N2O emission factor from low strength CWs, but decreased 37.2-43.7% of N2O emission factors for medium and high strengths compared with no rice straw (NRS) treatment. The RS application led to an average 10.7% increase in the potential denitrification rate, and simultaneously enhanced gene abundances of the total bacteria (16S rRNA), ammonia-oxidising archaea, ammonia-oxidising bacteria, nitrate reductase, and N2O reductase (nosZ) for all strengths CWs. The multiple regression model revealed N2O emissions were strongly related to water temperature, nitrate, chemical oxygen demand, and denitrification genes. The proportion of nosZ gene abundance in 16S rRNA was higher in RS (0.7-1.3%) than NRS (0.4-0.9%) for medium and high strengths, while an opposite trend was observed for low strength. The discrepancy was responsible for increasing or decreasing N2O emission by RS application among different strengths. These findings indicated the effectiveness of RS application to control N2O emissions from the surface flow CWs was related to the pollution level of wastewater.

Stimulation of optimized influent C:N ratios on nitrogen removal in surface flow constructed wetlands: Performance and microbial mechanisms.

Exploring optimal C:N ratio is necessary to ensure balanced microbial nitrification and denitrification in constructed wetlands (CWs), which has become an important management practice for more efficient nitrogen removal and sustainability of CWs. Surface flow constructed wetlands (SFCWs) vegetated with Myriophyllum aquaticum were designed to investigate the effects of five different influent C:N ratios (0:1, 2.5:1, 5:1, 10:1, and 15:1) on nitrogen removal performance and microbial communities over a 175-day experimental period. Compared to the influent C:N ratios of 0:1, higher NH4+-N, NO3--N, and total nitrogen (TN) removal efficiencies and lower NO3--N accumulation were observed at influent C:N ratios higher than 5:1. In addition, the highest TN removal efficiency (70.4%) and the lowest nitrous oxide emission flux (4.12 mg m-2 d-1) were obtained at the influent C:N ratio of 5:1. High-throughput sequencing revealed that influent C:N ratios altered the distribution and composition of microbial communities in the sediment, which resulted in a dynamic interplay between N-transforming functional microbes and NH4+-N and NO3--N removal. In particular, the dominant denitrifiers, including Desulfovibrio, Zoogloea, and Dechloromonas, were more abundant in the sediment with an influent C:N ratio of 5:1, which contributed to the high N removal rate. These findings may be used to screen for an optimum influent C:N ratio to maintain the sustainability of SFCWs with higher N removal efficiency.